1. Field of the Invention
The present invention relates to a device including a piezoelectric thin film and a method for producing such a device, and more specifically to an acoustic resonator and a micromachine switch usable in an radio frequency circuit of mobile communication terminals such as mobile phones, wireless LAN apparatuses and the like, and a method for producing the same.
2. Description of the Background Art
Components built in electronic apparatuses such as mobile phones and the like are demanded to be compact, lightweight, and small in loss and to provide high reliability. For fulfilling such demands, various types of devices including a piezoelectric thin film have been proposed. Devices expected to be compact, lightweight and small in loss are, for example, filters and micromachine switches using an acoustic resonator.
FIG. 13A is a cross-sectional view of an exemplary conventional acoustic resonator (see, for example, Japanese Laid-Open Patent Publication No. 60-68711). In this conventional acoustic resonator, a vibration section including a piezoelectric body 1 interposed between an upper electrode 2 and a lower electrode 3 is placed on a substrate 5. In the substrate 5, a cavity 4 is formed by partially etching the substrate 5 using a precision processing method performed from a surface on which the above-mentioned elements are not formed.
This acoustic resonator vibrates in a thickness direction of the piezoelectric body 1 when an electric field is applied thereto in the thickness direction by the upper electrode 2 and the lower electrode 3. Hereinafter, with reference to FIG. 13B through 13D, an operation of the acoustic resonator in the case where the thickness longitudinal vibration of an infinite flat plate is used will be described. FIG. 13B is a schematic isometric view of the acoustic resonator, which illustrates the operation thereof. FIG. 13C is a graph illustrating a frequency characteristic of admittance of the acoustic resonator. FIG. 13D shows an equivalent circuit configuration of the acoustic resonator.
When an electric field is applied between the upper electrode 2 and the lower electrode 3, an electric energy is converted into a mechanical energy by the piezoelectric body 1. The excited mechanical vibration is a vibration extending in a thickness direction, and extends and contracts in the same direction as the electric field. The acoustic resonator uses the resonating vibration in the thickness direction of the piezoelectric body 1 to operate by resonance at a frequency at which the thickness is equal to ½ wavelength. The thickness longitudinal vibration of the piezoelectric body 1 is guaranteed by the cavity 4. As shown in FIG. 13D, the equivalent circuit of the acoustic resonator includes a series resonance section including a capacitor C1, an inductor L1 and a resistor R1, and a capacitor C0 connected in parallel to the series resonance section. Therefore, the admittance of the acoustic resonator is maximum at the resonance frequency fr and is minimum at the anti-resonance frequency fa. fr=1/{2π·√(L1·C1)} and fa=fr·√(1+C1/C0).
FIG. 14 is an isometric view of an exemplary conventional micromachine switch using a piezoelectric effect (see, for example, Japanese Laid-Open Patent Publication No. 2003-217421). The conventional micromachine switch includes a signal line conductor 12 provided on a substrate 11, a driving shortcircuit mechanism 15 for shielding passage of radio frequency signals, and a piezoelectric body 16 which is driving means for giving a control signal to shift the driving shortcircuit mechanism 15.
With reference to FIG. 14, for shielding a signal, a voltage is applied to the piezoelectric body 16 as a control signal to put the signal line conductor 12 and ground conductors 13 into contact with a conductive layer 17 provided on a bottom surface of the driving shortcircuit mechanism 15. For allowing a signal to pass, no voltage is applied to the piezoelectric body 16.
In actuality, the conventional acoustic resonator described above has a vibration mode propagating along a plane parallel to the electrodes (transverse mode) in addition to the thickness direction vibration mode (longitudinal mode). In the acoustic resonator, a part of the vibration section is fixed to the substrate 5. Therefore, the vibration propagated parallel to the surface of the electrodes is reflected at the fixed position and thus becomes an unnecessary vibration. This unnecessary vibration causes spurious in the frequency characteristic.
For avoiding the spurious caused by the transverse mode, a technique, shown in FIG. 15, of forming a polygonal cavity in the acoustic resonator is proposed (see, for example, Japanese Laid-Open Patent Publication No. 2000-332568). Since the cavity of the acoustic resonator is polygonal, the vibration in the transverse mode, which is reflected at the fixed position, is propagated in a direction different from the direction of incidence. Thus, the spurious is reduced. Namely, appearance of the spurious in the frequency band of the thickness direction vibration mode of the acoustic resonator is avoided.
However, this technique has problems, for example, that the electrode and the cavity need to be designed for each acoustic resonator, and that redesigning is required each time the frequency or the impedance of the transmission path is changed.
The conventional acoustic resonator has a structure in which the local stress is concentrated on the piezoelectric thin film. Therefore, problems of layer delamination and cracks occur during the production.
For solving these problems, an acoustic resonator shown in FIG. 16 is disclosed (see, for example, Japanese Laid-Open Patent Publication No. 2005-45694). In this acoustic resonator, at a step portion of the interface between a piezoelectric film 32 and a lower electrode 31, which corresponds toward the edge of a gap V, a plurality of planes which are not parallel to the surface of a substrate 30 and have different angles α, β and γ with respect to the surface of the substrate 30 are stacked from the substrate 30 toward the top of the gap V. Owing to such a structure (air bridge), the local stress is prevented from being concentrated on the piezoelectric film 32.
However, with this technique, a support layer 40 needs to have a complicated shape in order to provide a plurality of different angles of the interface of the piezoelectric film 32 and the lower electrode 31 with respect to the surface of the substrate 30. This has a problem of, for example, complicating the production method although alleviating the stress concentration.
In the conventional micromachine switch described above, the driving shortcircuit mechanism 15 and a support section 9 are connected perpendicular to each other. Therefore, when the driving shortcircuit mechanism 15 is shifted mechanically, a stress is concentrated on the connection point and thus the mechanical reliability is lowered.